The present invention relates to aqueous surface treatment chemicals for forming hydrophilic coatings on aluminum members such as heat exchanger fins, etc., a bath containing such surface treatment chemicals, and a method of surface-treating an aluminum member with such an aqueous surface treatment bath. More particularly, it relates to hydrophilic surface treatment chemicals for forming hydrophilic coatings with good corrosion resistance and high hydrophilic nature and also with no or extremely reduced odor on aluminum members such as fins of heat exchangers, a bath containing such a surface treatment chemicals and a method of treating them.
Aluminum and its alloys are light and have good workability and heat conductance, so that they are widely used for fins of heat exchangers. Recently more and more air conditioners have been used not only for cooling but also for warming and dehumidification. In heat exchanger parts of these air conditioners, aluminum alloy fins are generally used.
However, it is observed that moisture tends to be condensed and deposited as water droplets on the fin surfaces of air conditioners during cooling operations. If the fin surface is water-repellent, this condensed water tends to be deposited in a hemispherical form on the fin surface or forms bridges between the fins, preventing smooth air flow, which in turn increases resistance of air flow, thereby decreasing heat exchange efficiency.
In addition, although aluminum and its alloys are essentially excellent in corrosion resistance, it is likely that the condensed water remaining on the aluminum fin surfaces for a long period of time functions like an oxygen concentration cell, and that contaminants and sulfur oxides in the air are absorbed and concentrated in the condensed water. As a result, a hydration reaction and a corrosion reaction are accelerated. Those produced by the corrosion are accumulated on the aluminum fin surfaces, which not only deteriorate heat exchange performance, but also are blown out of the air conditioners as white fine powders together with hot air during the warming operation in the winter season.
To solve these problems, various attempts have been made to form on aluminum fin surfaces coatings for improving their corrosion resistance and for increasing their hydrophilic nature.
For instance, Japanese Patent Laid-Open Nos. 55-12375 and 56-56572 disclose evaporators having fins coated with synthetic resins comprising hydrophilic amides. A water-soluble nylon is chosen as a resin for coating the fins of these evaporators.
Japanese Patent Laid-Open No. 61-250495 discloses an aluminum fin having a chemical primer coating and a coating which comprises organic polymer materials such as water-soluble polyamides showing cationic characteristics in an aqueous solution.
Further, Japanese Patent Laid-Open No. 63-57674 discloses hydrophilic surface treatment chemicals comprising a water-soluble resin such as water-soluble nylon, an alkali metal silicate and an amino alcohol.
Furthermore, Japanese Patent Laid-Open No. 62-132970 discloses surface treatment chemicals for treating fins of a heat exchanger, which comprises (A) an adduct of (a) at least one of urea, thiourea and guanidine and (b) formalin, or a condensate of the adduct, and (B) a water-soluble nylon. Japanese Patent Laid-Open No. 62-176578 discloses a method of surface treatment by using the above chemicals.
However, any one of the coatings disclosed in the above references fails to have good hydrophilic nature and sufficient effect of preventing odor.
An aluminum-made heat exchanger provided with a coating comprising a water-insoluble germicide to kill microorganisms in water droplets condensed on the fins, etc., thereby preventing unacceptable odor caused by the microorganisms is disclosed in Japanese Patent Laid-Open No. 60-50397, but this coating is still insufficient in hydrophilic nature and odor-preventing effect.
As mentioned above, the conventional surface treatment technologies for heat exchangers fail to provide coatings which have not only sufficient hydrophilic nature. corrosion resistance and strength but also excellent odor-preventing effect.